This invention relates to a method for producing an elongated composite glass body by melting at least two starting materials of different compositions, feeding the starting materials to a drawing die disposed at the bottom of an externally heated melting crucible, and continuously drawing the melts vertically out of the drawing die to form a composite body laminated transverse to the drawing direction. The invention also relates to an apparatus for producing an elongated composite glass body, comprising an externally heated melting crucible having a drawing die at the bottom thereof and a feed device for feeding at least two starting materials of different compositions into the melting crucible.
A method and apparatus of this type are known, for example, from German Patent No. DE-OS 27 03 706. This patent describes a method for the continuous production of an optical waveguide for optical communication, which waveguide consists of a core glass and an outer glass cover enclosing the core glass and having a lower refractive index than the core glass. In this method, the optical waveguide is drawn from a so-called double crucible. The double crucible consists of an externally heated outer crucible which contains the outer glass melt and in which is concentrically disposed an inner crucible which contains the core glass melt and whose side walls project above the upper surface of the outer glass melt. The bottom of the outer crucible is provided with a drawing die. An outlet opening is provided in the bottom of the inner crucible that is spaced vertically above the drawing die, the outlet opening of the inner crucible thus projecting into the outer glass melt, so that the core glass melt passing out of the inner crucible will be completely surrounded by the outer glass melt as they pass out of the drawing die of the outer crucible. The inner and outer crucibles are fed with the starting materials for the composite body independently and separately from one another by continuously adding glass rods composed of the starting materials to the respective melts. Both the inside and the outside crucible are made of sheet platinum. It is known that platinum starts to dissolve in glasses at temperatures above 900.degree. C. In addition, it is possible that platinum particles will be detached from the crucible walls and be entrained by the melt flowing along the crucible walls.
In European patent application No. EP-A2 0 185 362 an apparatus is described for continuously producing optical fibers composed of two different materials, in which the glass-forming starting materials for the composite body and doping agents in reactive, liquid form are fed to a double crucible at room temperature. The inner and outer crucibles of the double crucible are disposed concentrically to one another and their conically tapering outlet openings are disposed vertically above one another. The inner crucible is movable in the vertical direction inside the outer crucible. The top part of the double crucible, in which the liquid starting materials react with one another to form a partly solidified mass, is surrounded by a hardening furnace, the temperature of which increases, from top to bottom, from room temperature to hardening temperature. The lower part of the outer crucible extends into a second furnace, which likewise has a temperature gradient from top to bottom, with a maximum temperature in the bottom part sufficiently high to soften the composite mass to such an extent that a composite fiber can be drawn from it out of a drawing die at the bottom of the outer crucible.
These known methods for the production of a composite body of glass have the common feature that two starting materials are melted or collected in separate crucibles, one crucible being disposed as an inner crucible inside the other and which has at least one outlet opening projecting into the second, outer crucible. Particularly at the high temperatures encountered and with aggressive starting materials, this can lead to reactions with the material of the inner crucible, which may for example result in abrasion of the inner crucible or the production of inhomogeneities, such as, at the surface of contact between the inner crucible and the starting materials for the composite body. After the composite body has been drawn, these inhomogeneities or impurities intensify at the interface between the different starting materials, where they are usually particularly troublesome.
In the known methods, the spatial distribution of the two starting materials inside the composite body can be adjusted within certain limits by adjusting the distance between the outlet opening of the inner crucible and the drawing die of the outer crucible and by varying the geometry of the die openings. The double crucible designs needed for this purpose, or the conversion procedures required are, however, expensive. In the dimensioning of the die openings, taking into account the viscosities of the melts during drawing, care must also be taken to ensure that they do not clog the outlet opening of the inner crucible disposed concentrically inside the outer crucible and therefore inside a relatively colder region of the melt.
If the inner and outer crucibles are kept at a constant temperature and in a fixed spatial relation to one another, the spatial distribution of the starting materials inside the composite body will be dependent on the filling levels of the melts in the respective crucibles (with the same pressure above the melt). If the spatial distribution of the starting materials in the composite body is to be constant, the problem then arises of keeping the height of the melt surfaces at a constant level or in a constant proportion to one another.
The problem underlying the invention therefore is to provide a method of the type first mentioned above for producing a composite glass body, wherein a homogeneous interface free from impurities is formed between the starting materials in the composite body, and in which the path of the composite body interface between the materials of the composite body can be shaped in a simple manner and with flexibility, and also that of making available a simple, trouble-free apparatus for producing such composite bodies, which is particularly suitable for producing composite bodies at high temperatures and which permits the shaping of the composite body interface in a simple manner and with a great deal of flexibility.